MCM-22 molecular sieve has a MWW topological structure, and two sets of independent ten-membered ring pore systems which do not communicate with each other, one of which comprises two-dimensional sinusoidal pores having a cross section of approximately ellipse shape with a pore size of 4.1 Å×5.1 Å, the other of which comprises twelve-membered ring supercages of approximately cylindrical shape with a size of 7.1 Å×7.1 Å×18.2 Å, which supercage communicates with the outside through a slightly distorted ten-membered ring opening (4.0 Å×5.5 Å). Moreover, the MCM-22 molecular sieve has bowl shaped twelve-membered ring semi-supercages located on the external surface of the crystal.
Based on the study of the structure of the MCM-22 molecular sieve by found that there is some significant difference in structure between a MCM-22 molecular sieve precursor powder without removing templates by calcination (MCM-22(P)) and a MCM-22 molecular sieve after calcination (MCM-22(C)). Upon comparison of the lattice parameters thereof, there is found that MCM-22(P) and MCM-22(C) share the same lattice parameter a(b), both being 1.427 nm, while MCM-22(C) has a lattice parameter c of 2.52 nm, and MCM-22(P) has a lattice parameter c of 2.68 nm. The same lattice parameters a(b) indicate that the layered structure already formed with the MCM-22(P) does not change during calcination. On the basis of this, the mechanism under which a MCM-22(P) is converted into a MCM-22(C) by calcination to remove templates is deduced as: first of all, the templates between different layers desorb and decompose at elevated temperatures, and at the same time, silanol groups (Si—OH) are formed on the surface of the layered structure, and then, the Si—OH groups on the surface dehydrate by condensation into Si—O—Si bonds, whereby connecting adjacent layered structures into a multi-layered structure.
The MCM-22 molecular sieve generally presents as a flake or thin plate morphology, with a size of about 2 μm, a thickness of from more than 10 to tens nm. Upon study of the MCM-22 molecular sieve crystal by transmission electron microscope, it is found that the flake crystal of the MCM-22 molecular sieve has a multi-layered structure made by connecting numbers of “elementary building layer structure” having a thickness of about 2.5 nm with oxo bridging bonds. Depends on the thickness of the molecular sieve crystal, the multi-layered structure may be made of different numbers of elementary building layer structure, generally more than 5, or even up to 10 or more.
In the multi-layered structure of the MCM-22 molecular sieve, the two sets of ten-membered ring pore locate inside the layered structure and between two adjacent elementary building layer structures respectively, imposing strict restriction on molecular diffusion, while the twelve-membered ring semi-supercages locating on the surface of the crystal facilitate molecular diffusion. In fact, when a MCM-22 molecular sieve is used to catalysis an alkylation reaction in liquid phase of benzene and ethylene, the reaction is found to only occur in the bowl shaped semi-supercages on the crystal surface, while the ten-membered ring pores inside and between the layered structure are inaccessible to the reaction.
To make more effective use of these ten-membered ring pores of a MCM-22 molecular sieve between different layers, and to expose as much as possible twelve-membered ring supercages inside its structure, for example, WO9717290 developed a process comprising swelling a MCM-22(P), and then ultrasonicating the swelled MCM-22(P) to destroy the interaction between different elementary building layer structures whereby taking apart the elementary building layer structures, so as to obtain an ITQ-2 molecular sieve having only one single elementary building layer structure (also called as single layered structure). This ITQ-2 molecular sieve has been identified as a novel molecular sieve having a MWW topological structure in this field. Due to this specific single layered structure, the ITQ-2 molecular sieve has a crystal thickness of only about 2.5 nm. As compared with the MCM-22 molecular sieve, the ITQ-2 molecular sieve is retained with only the ten-membered ring pores inside the layer, while the ten-membered ring pores comprising cylindrical twelve-membered ring supercages between adjacent layers are totally destroyed, wherein the supercage is divided into two bowl shaped semi-supercages and rendered totally open. Then, the ITQ-2 molecular sieve is significantly improved with its external specific surface area (generally about 700 m2 g−1 vs. only about 100 m2 g−1 for a MCM-22 molecular sieve), showing no restriction to molecular diffusion, whereby exhibiting significantly superior diffusion performances as compared with the MCM-22 molecular sieve. However, the ITQ-2 molecular sieve has only one single layered MWW topological structure, which indicates that the three dimensional structure of the MWW material has been totally destroyed by dividing one cylindrical complete supercage into two bowl shaped semi-supercages, leading to transformation of the B acid center inside the supercage into a L acid center (IR study of the acidity of ITQ-2, an “all-surface” zeolitic system, Journal of Catalysis, 214 (2003), pp. 191-199). Specifically, the acid center of the ITQ-2 molecular sieve is mainly a L acid, which is similar to the acid property of a mesoporous material. Taking into consideration of the fact that the ITQ-2 molecular sieve has a comparable specific surface area to that of a mesoporous material, the ITQ-2 molecular sieve may act more like a mesoporous material rather than a crystalline microporous molecular sieve. For these reasons, for the ITQ-2 molecular sieve, due to the destruction of its microporous structure, the ITQ-2 molecular sieve exhibits diffusion and adsorption performances comparable to a mesoporous material, and is deprived of properties, typically associated with a crystalline microporous molecular sieve, such as thermal stability, hydrothermal stability or catalytic shape selectivity. In view of this, the ITQ-2 molecular sieve is effective in converting a reactant (i.e. having a high reactant converting capability) in a reaction which is more tolerable to the intensity of the acid center or severely restricted by molecular diffusion, but ineffective in selective generation of an aimed product (i.e. having a poor product selectivity). For example, in an alkylation reaction in liquid phase between benzene and ethylene, which is a reaction in need of mid-strong or strong acids as the catalyst, due to its weaker acidity, as compared with the MCM-22 molecular sieve, the ITQ-2 molecular sieve is less applicable to this reaction. On the contrary, the Beckmann rearrangement reaction of cyclohexanone oxime to produce caprolactam is a typical diffusion-restricted reaction. In this reaction, cyclohexanone oxime is greater in molecular size than the ten-membered ring openings of the MCM-22 molecular sieve, while the product caprolactam is even much greater, for this reason, the reaction mainly occurs in the bowl shaped twelve-membered ring semi-supercages on the external surface of the crystal. In this regard, as compared with the MCM-22 molecular sieve, the ITQ-2 molecular sieve has much more opened twelve-membered ring semi-supercages, thus exhibiting significantly superior catalytic performances over the MCM-22 molecular sieve. Further, the ITQ-2 molecular sieve has to be produced in a rather complicated manner, necessarily involving at least steps of producing a MCM-22 molecular sieve precursor, and swelling and ultrasonication treatment of the thus produced MCM-22 molecular sieve precursor. In this context, the process for producing the ITQ-2 molecular sieve suffers from the problem of massive energy and material consumption. Besides, the swelling conditions are so harsh that the crystalline structure of the molecular sieve are severely destroyed in most cases, leading to a massive loss of silicon into the liquid phase, and as a consequence, it is very difficult for the ITQ-2 molecular sieve to have a yield of more than 50%. Further, during the production of the ITQ-2 molecular sieve, a swelling agent like hexadecyl trimethyl ammonium bromide (CTAB) has to be used. This swelling agent acts also like a surfactant, which leads to organization of silicon species in the liquid phase around its micelle into a mesoporous material and then isolation from the liquid phase, and is further mixed into the produced ITQ-2 molecular sieve, whereby reducing the purity of the ITQ-2 molecular sieve. A mesoporous material has much greater specific surface area than a microporous molecular sieve, and this contamination by the mesoporous material as an impurity may increase the total specific surface area and external specific surface area of the final product. However, due to its non-crystalline nature, the mesoporous material is characterized by a poor hydrothermal stability and further a poor resistance to both acid and alkali. Unfortunately, the prior art fails to develop any process to effectively remove these mesoporous materials from the produced ITQ-2 molecular sieve by now. Due to the avoidless presence of this mesoporous materials as the impurity, the catalytic performances of the ITQ-2 molecular sieve has been significantly compromised.
In practical use, in addition to good catalytic performances (including the reactant converting capability and the product selectivity), the molecular sieve is further required to has good regeneration performance. Due to its one single layered structure and the unavoidable contamination by the mesoporous material as the impurity, if the ITQ-2 molecular sieve is to be regenerated in a conventional manner, e.g. by calcination at elevated temperatures, Si—OH and Al—OH groups on its surface will dehydrate by condensation, followed by sintering and fusing of the framework structure of the molecular sieve, leading to burying and blocking of pores and loss of the active centers, resulting in significant reduction of its catalytic performances and impossible to be recovered to the same level of a fresh catalyst. Due to these problems, the ITQ-2 molecular sieve has been known for nearly 20 years since the first report of same, and observed with high performances thereof by many researchers, however, fails to be commercially used up to now.
Therefore, there is still a need for a novel molecular sieve, which shows a reactant converting capability comparable to the ITQ-2 molecular sieve even for a typical diffusion-restricted reaction, and is deprived of the problems in association with the ITQ-2 molecular sieve.